In recent years, a MIMO scheme is actively being used as a method for effectively utilizing limited radio frequency resources. According to a transmission scheme using the MIMO scheme, the transmitting side and receiving side simultaneously and spatially multiplex and transmit a plurality of synchronized radio streams using a plurality of antennas. Therefore, the transmission scheme using the MIMO scheme can improve the frequency utilization efficiency compared to a case where single signals are transmitted, depending on situations of radio transmission channels.
According to the MIMO scheme, the receiving side can also separate a plurality of spatially combined streams by performing signal processing using a plurality of antennas (see Patent Literature 1). However, depending on the situations of radio transmission channels, influences of cross interference among streams are so large that the transmission error rate may deteriorate.
To reduce these influences, the MIMO scheme generally employs a method of performing error correction using soft decision values after demapping. This method multiplies streams after MIMO separation by a coefficient proportional to signal-to-noise power of the separated streams or the square root thereof or the like. This makes it possible to maintain original reception amplitude information, maximize the performance of a soft decision error correcting decoder and suppress influences of cross interference among streams.
Furthermore, the MIMO scheme provides a method of acquiring a coefficient proportional to signal-to-noise power of streams after the above-described MIMO separation using parameters obtained in the process of calculating an inverse matrix of transfer factors without the need for separately providing envelope generation (see Patent Literature 2).
Furthermore, Non-Patent Literature 1 describes details of this calculation. This calculation will be described briefly below. When channel characteristic H between N transmitting antennas and N receiving antennas is expressed as shown in equation 1, inverse matrix G of channel characteristic H is expressed as shown in equation 2.
                    (                  Equation          ⁢                                          ⁢          1                )                                                            H        =                  (                                                                      h                  11                                                                              h                  21                                                            …                                                              h                                      N                    ⁢                                                                                  ⁢                    1                                                                                                                        h                  12                                                                              h                  22                                                            …                                                              h                                      N                    ⁢                                                                                  ⁢                    2                                                                                                      ⋮                                            ⋮                                            ⋱                                            ⋮                                                                                      h                                      1                    ⁢                    N                                                                                                h                                      2                    ⁢                    N                                                                              …                                                              h                  NN                                                              )                                    [        1        ]                                (                  Equation          ⁢                                          ⁢          2                )                                                                                                G              =                            ⁢                                                (                  H                  )                                                  -                  1                                                                                                        =                            ⁢                                                1                                                          H                                                                      ⁢                                                                           t                                    ⁢                                      [                                                                                            (                                                      -                            1                                                    )                                                                          i                          +                          j                                                                    ⁢                                                                                                H                          ij                                                                                                              ]                                                                                                                          =                            ⁢                              (                                                                                                    g                        11                                                                                                            g                        21                                                                                    …                                                                                      g                                                  N                          ⁢                                                                                                          ⁢                          1                                                                                                                                                                        g                        21                                                                                                            g                        22                                                                                    …                                                                                      g                                                  N                          ⁢                                                                                                          ⁢                          2                                                                                                                                                ⋮                                                              ⋮                                                              ⋱                                                              ⋮                                                                                                                          g                                                  1                          ⁢                          N                                                                                                                                    g                                                  2                          ⁢                          N                                                                                                            …                                                                                      g                        NN                                                                                            )                                                                        [        2        ]            
where, |Hij| represents a minor determinant of hij and t represents a transpose matrix.
A spatially multiplexed signal is separated into a plurality of streams using a feedforward type inter-channel interference canceller (linear interference canceller) of a ZF (Zero Forcing) criterion or MMSE (Minimum Mean Square Error) criterion or the like. An n-th stream after the stream separation is multiplied by weighting factor wn (see equation 3).
                    (                  Equation          ⁢                                          ⁢          3                )                                                                      w          n                =                  1                                                    ∑                                  l                  =                  1                                N                            ⁢                                                                                      g                    ln                                                                    2                                                                        [        3        ]            
FIG. 1 is a block diagram of a receiving apparatus that performs the above-described processing. In FIG. 1, each stream MIMO-separated through a matrix calculation is subjected to demapping for linear modulation such as QAM (Quadrature Amplitude Modulation). Since the amplitude and phase are equalized through the matrix calculation, the demapping section can make a decision based on a threshold set on the x-axis or y-axis on a phase space diagram. By calculating the distance from the threshold for each bit, an approximate likelihood is calculated.
FIG. 2 is an example where a constellation for 16QAM is shown on a phase space diagram. For example, of signal points corresponding to a bit stream of four bits of (b0, b1, b2, b3), the probability that the b1 bit may be 1 is high at signal points located in the shaded area in the figure and the probability that the b1 bit may be 0 is high at signal points located outside the shaded area. Therefore, when the x coordinate of a signal point to be decided is +, the distance from threshold +2 is calculated as a likelihood of the b1 bit and when the x coordinate of a signal point to be decided is −, the distance from threshold −2 is calculated as a likelihood of the b1 bit. Thus, when the modulation scheme is 16QAM, likelihoods are calculated for the four bits respectively.
Furthermore, when bit-by-bit interleaving is applied on the transmitting side, a demapping section converts a likelihood for each stream from parallel to serial and outputs the converted likelihood. A multiplication section multiplies the likelihood outputted from the demapping section by weighting factor wn and the likelihood after the multiplication is rearranged by a deinterleaver. When convolutional coding is performed on the transmitting side, a soft decision Viterbi circuit performs error correction on the likelihood after the rearrangement and obtains a decoded word. The decoded word is outputted to a MAC (Media Access Control) section that performs protocol processing.
Thus, according to the above-described prior art, an inverse matrix operation section and a weighting factor calculation section perform a division operation (see equation 2 and equation 3).
When a frame made up of a preamble part and a data part as in the case of IEEE802.11n is used, the MIMO receiving apparatus performs channel estimation using the preamble part transmitted at the start of the frame and obtains a channel matrix. The MIMO receiving apparatus then adopts a configuration of calculating an inverse matrix of the channel matrix obtained (transfer factor inverse matrix in Patent Literature 2), temporarily saving the inverse matrix and repeatedly using the saved inverse matrix in the data part transmitted after the preamble part. That is, a general MIMO receiving apparatus performs a division operation necessary to calculate an inverse matrix only during processing of the preamble part.
Furthermore, the MIMO receiving apparatus adopts a CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance) scheme. The MIMO receiving apparatus that adopts the CSMA/CA scheme must send back an Ack (ACKnowledgement) signal to transmit the fact that the reception has been successful to the transmitting side within a defined time. Therefore, the MIMO receiving apparatus needs to finish the processing of the preamble part as quickly as possible. Thus, the division operation for inverse matrix calculation needs to be processed at high speed though it is performed only once per frame.